1. Field of the Invention
This invention pertains to the automated, semi-continuous identification of specific organic compounds at the molecular level, and the determination of the concentration of these particle-bound organic compounds in airborne particles.
2. Description of the Related Art
Organic matter is a major constituent of airborne particulate matter (PM), and may comprise 20-50% of the mass of those particles with diameters smaller than 2.5 μm (PM2.5). (See, for example, Chow, J. C., Watson, J. G., Lowenthal, D. H., Solomon, P. A., Magliano, K. L., Ziman, S. D., Richards, L. W., 1993. PM10 and PM2.5 compositions in California's San Joaquin Valley. Aerosol Science and Technology 18: 105-128; Schauer J. J. and Cass G. R. (2000) Source apportionment of wintertime gas-phase and particle phase air pollutants using organic compounds as tracers, Environ. Sci. Technol. 34: 1821-1832; Kim, B. M., Teffera, S., Zeldin, M. D., 2000. Characterization of PM2.5 and PM10 in the South Coast air basin of southern California: Part 1n—spatial variation. J. Air and Waste Management Assoc. 50: 2034-2044; Christoforou, C. S., Salmon, L. G., Hannigan, M. P., Solomon, P. A., Cass, G. R., 2000. Trends in fine particle concentration and chemical composition in southern California. J. Air and Waste Management Assoc. 50: 43-53; NARSTO. (2003). Particulate Matter Sciences for Policy Makers, A NARSTO Assessment, Chapter 6.
The chemical composition of organic matter is complex and largely not understood. Many hundreds of organic compounds have been identified through chromatography and mass spectrometry techniques including alkanes, substituted phenols, alkanals, sugar derivatives, aromatic polycyclic hydrocarbons, mono- and di-carboxylic acids.
While the compounds which have been identified comprise only a fraction of the total organic mass, those that are quantified serve as valuable tracers for sources. For example, hopanes, which are remnants of the biological material from which petroleum originated, serve as a unique tracer for fossil fuel combustion. Levoglucosan is a product of the breakdown of cellulose, and is a unique tracer for wood combustion. Biogenic alkanes are distinguished from fossil-derived alkanes through a carbon preference number that reflects the predominance of odd-carbon number alkanes in plant waxes. These differences in organic compound composition have been used to determine the relative contribution of various source types to primary ambient organic matter (See, for example, Schauer J. J. and Cass G. R. (2000) Source apportionment of wintertime gas-phase and particle phase air pollutants using organic compounds as tracers, Environ. Sci. Technol. 34: 1821-1832; Fraser M. P., Kleeman, M. J., Schauer, J. J., Cass, G. R. (2000) Modeling the Atmospheric Concentrations of Individual Gas-Phase and Particle-Phase Organic Compounds, Environ. Sci. Technol., 34: 1302-1312; Fine, P. M., Cass G. R., Simoneit, B. R. T. (2001) Chemical characterization of fine particle emissions from fireplace combustion of woods grown in the northeastern United States, Environ. Sci. Technol. 35: 2665-2675; Yue, Z. and Fraser, M. P. (2003a). Characterization of non-polar organic fine particulate matter in Houstion, Tex. Submitted for publication; Yue, Z. and Fraser, M. P. (2003b). Polar organic compounds measured in Fine Particulate matter during TexAQS 2000. Submitted for publication).
A substantial limitation in the use of organic marker compounds for source identification is the difficulty of the analyses. The identification and quantification of organic matter at the compound level involves integrated sample collection by filtration or impaction with subsequent extraction and analysis by liquid or gas chromatography. Generally large samples are required, and analyses are time-consuming and expensive. These methods have provided valuable insight and guidance in the understanding of airborne organic matter, but are limited by their poor time resolution, intensity of manual efforts, and cost.
Over the last decade, several types of particle beam mass spectrometry methods have emerged for the study of ambient particles, including two methods that are now available commercially (Noble C. A. and Prather K. A. (1996) Real time measurement of correlated size and composition profiles of individual atmospheric aerosol particles, Environ. Sci. Technol. 30: 2667-2680. Jayne, J. T., Leard D. C., Zhang X-F. Davidovits P., Smith K. A., Kolb, C. E., Worsnop D. R. (2000) Development of an aerosol mass spectrometer fro size and composition analysis of submicron particles, Aerosol Sci. Technol. 33: 49-70). These instruments provide a wealth of real-time data on aerosols and on single particle composition. Yet all face considerable challenges in identifying and quantifying the complex mix of organic compounds found in ambient aerosols. The deconvolution of mass spectra from multiple organic compounds remains a daunting challenge.
Particle-beam mass spectrometers are fast and automated, but do not provide the necessary compound separation for quantitative assay of organic compounds at the molecular level. Filter measurements, with laboratory extraction, chromatographic separation and mass spectrometric analyses provide identification and quantification at the molecular level, but are labor intensive, costly and slow to yield results.
Hence, a high throughput system which provides separation of organic compounds prior to their identification and quantification by a mass spectrometric method, would be highly valuable.